Group 11-13 and Group 11-16 compound semiconductors having a chalcopyrite crystal structure or cubic or hexagonal Group 12-16 compound semiconductors have a large absorption coefficient for light in the wavelength range from visible to near infrared, and therefore, they are expected as a material for high-efficiency thin-film solar cells. Representative examples thereof include a Cu(In,Ga)Se2 (hereinafter referred to as CIGS) and a CdTe.
In the CIGS thin-film solar cell, a soda lime glass which is inexpensive and has a thermal expansion coefficient close to that of the CIGS compound semiconductor, has been used as the substrate to obtain the solar cell.
Also, for obtaining high efficiency solar cells, a glass material resistant to a heat treatment with a high temperature has been proposed (see, Patent Document 1).
Furthermore, use of an alkali glass substrate containing an alkali metal oxide has been proposed as a glass substrate for a TFT panel (for “a-Si TFT panel”) (see Patent Documents 1 and 2). This is due to that a heat treatment in a TFT panel manufacturing step, that has conventionally been conducted at from 350 to 450° C., is becoming possible to be conducted at a relatively low temperature (from about 250 to 300° C.).
A glass including an alkali metal oxide generally has high thermal expansion coefficient. For this reason, B2O3 having the effect of reducing a thermal expansion coefficient is generally contained for the purpose of achieving a thermal expansion coefficient preferable as a glass substrate for a TFT panel (Patent Documents 2 and 3).
However, in the case where a glass has a composition containing B2O3, B2O3 volatilizes when the glass has been melted, particularly in a melting step, a fining step and a float molding step. As a result, the glass composition tends to become heterogeneous. In the case where the glass composition becomes heterogeneous, such a glass composition affects flatness when molding into a plate shape. A glass substrate for a TFT panel is required to have high degree of flatness in order to maintain a distance between two glasses interposing liquid crystal therebetween, that is, a cell gap, constant for the purpose of ensuring the quality indicated. For this reason, after the glass has been molded into a plate glass by a float process, the surface of the plate glass is polished in order to secure given flatness. In the case where given flatness is not obtained in the plate glass after molding, the time required for a polishing step is prolonged, leading to decrease in productivity. Furthermore, considering environmental load due to the volatilization of B2O3, the content of B2O3 in a molten glass is better to be lower content.
However, in the case where the B2O3 content is low, it has been difficult to decrease a thermal expansion coefficient down to the thermal expansion coefficient preferred as a glass substrate for a TFT panel, and to obtain given Tg and the like while suppressing the increase in viscosity.